People always joke about the Marauders thinking a stray dog is really Sirius but imagine that the Marauders are exploring the Forbidden Forest and they split up and all of a sudden, a deer comes over and nudges Sirius. And he’s just like, “Oh, hey, mate.” And the deer just nudges him again. And they finally decide to head back to the castle but the deer is having none of it so they literally have to drag him out of the forest. Only they’re panicking because he’s not changing back and come on James, this isn’t funny, you know we can’t get caught. And so they start to worry that something is wrong and he can’t change back. And so they smuggle him back into the castle. And by smuggle I mean, they throw the Invisibility Cloak over him but since he’s a deer, it doesn’t really cover much. And that’s the story of how Professor McGonagall looks up and sees three boys shoving a headless deer past her doorway and she promptly decides that it’s not her problem.

So they get the deer to the common room and it’s only then that they begin to speculate that maybe this deer isn’t James after all. But the deer just keeps doing really vague things that could be James. And they spend the rest of the afternoon having to decide if this really is James and if it is, if he’s messing with them or if something’s wrong, and if he’s not, did they just kidnap a deer?

from
Short Stories from Hogwarts of Heroism, Hardship and Dangerous Hobbies

An Animagus is a witch or wizard who can transform at will into an animal. While in their animal form, they retain most of their ability to think as a human, their own sense of identity and their memories. They will also retain normal human life expectancy, even if they take their animal form for long periods of time. However, feelings and emotions are simplified and they will have many animal desires, feeding off whatever their animal body craves, rather than demanding human food.

It is immensely difficult to change oneself into an Animagus and the process, which is complex and time-consuming, can go dramatically wrong. As a result, it is believed that fewer than one in a thousand witches or wizards are Animagi.

An Animagus has a great potential advantage in the spheres of espionage and crime. For this reason, an Animagus Registry exists on which all Animagi are expected to log their personal details and the precise appearance of their transformed self. It is usually the case that distinctive markings or disabilities belonging to the human body will transfer to the animal self. Failure to enter oneself onto the Registry may result in a stretch in Azkaban.

When the process of becoming an Animagus goes wrong, it often goes seriously wrong. Impatience with the long and complicated process is generally at the root of such disasters, which usually take the form of horrible half-human, half-animal mutations. There is no known cure for such mistakes and those who make them are often forced to live out their days in their pitiable condition, being unable to become fully animal or fully human.

Talent in both Transfiguration and Potions is necessary to become an Animagus. No responsibility can be taken for any physical or mental problems resulting from following these instructions.

For the space of one entire month (from full moon to full moon), a single leaf from a Mandrake must be carried constantly in the mouth. The leaf must not be swallowed or taken out of the mouth at any point. If the leaf is removed from the mouth, the process must be started again.

Remove the leaf at the full moon and place it, steeped in your saliva, in a small crystal phial that receives the pure rays of the moon (if the night is cloudy, you will have to find a new Mandrake leaf and begin the whole process again). To the moon-struck crystal phial, add one of your own hairs, a silver teaspoon of dew collected from a place that neither sunlight nor human feet have touched for a full seven days, and the chrysalis of a Death’s-head Hawk Moth. Put this mixture in a quiet, dark place and do not look at it or otherwise disturb it until the next electrical storm.

While waiting for the storm, the following procedure should be followed at sunrise and sundown. The tip of the wand should be placed over the heart and the following incantation spoken: ‘Amato Animo Animato Animagus.’

The wait for a storm may take weeks, months or even years. During this time, the crystal phial should remain completely undisturbed and untouched by sunlight. Contamination by sunlight gives rise to the worst mutations. Resist the temptation to look at your potion until lightning occurs. If you continue to repeat your incantation at sunrise and sunset there will come a time when, with the touch of the wand-tip to the chest, a second heartbeat may be sensed, sometimes more powerful than the first, sometimes less so. Nothing should be changed. The incantation should be uttered without fail at the correct times, never omitting a single occasion.

Immediately upon the appearance of lightning in the sky, proceed directly to the place where your crystal phial is hidden. If you have followed all the preceding steps correctly, you will discover a mouthful of blood-red potion inside it.

It is essential to move, at once, to a large, secure place where your transformation cannot cause alarm or place you in physical danger. Place your wand-tip against your heart, speak the incantation ‘Amato Animo Animato Animagus,’ and then drink the potion.

If all has gone correctly, you will feel a fiery pain and an intense double heartbeat. Into your mind will come the shape of the creature into which you are shortly to transform. You must show no fear. It is too late, now, to escape the change you have willed.

The first transformation is usually uncomfortable and frightening. Clothing and items such as glasses or jewellery meld to the skin and become one with fur, scales or spikes. Do not resist and do not panic or the animal mind may gain the ascendancy and you could do something foolish, such as try to escape through a window or charge a wall.

When your transformation is complete you should find yourself physically comfortable. You are strongly advised to pick up your wand at once, and hide it in a place of safekeeping, where you will be able to find it when you regain a human form.

To return to a human form, visualise your human self as clearly as you can. This should be sufficient, but do not panic if the transformation does not occur immediately. With practice, you will be able to slip in and out of your animal form at will, simply by visualising the creature. Advanced Animagi can transform without wands.

Generally wizards prefer to have their clothes Transfigure with them, to escape the embarrassment of reappearing naked. However, it is possible to leave clothes behind if one wishes to give the impression of having gone for a bath or something similar. The longer a witch or wizard has been an Animagus, the better they will become at choosing the precise form of their transformations.

The animal into which one turns, if an Animagus, seems always to be that which becomes the Patronus. There is no known instance of the Animagus form changing to match the Patronus if the latter changes, but the Animagus who can also produce a Patronus is highly unusual and no study has ever been done on sufficient numbers to draw firm conclusions.

I think that the Marauders often orchestrated little skits where Sirius would come bounding into school in his animagus form so that he could steal parchment out of one of the Marauders (mainly James) who hasn’t done their homework and they can start yelling,
“A DOG IS STEALING MY HOMEWORK! A DOG IS EATING MY HOMEWORK! ARGH! OH MERLIN WHAT A TRAGEDY IS THIS?! MY BEAUTIFULLY COMPLETE HOMEWORK. GONE! FOREVER!” They yell it so loudly that their professors hear and let them off and one day Sirius wants to get out of doing his homework so James does it instead - everyone completely freaks out as a huge stag gallops into school and snatches some paper out of a grinning Sirius’ hand

okay but what if sirius got bored one day so he just. transformed into his animagus form and followed james and lily on one of their dates and it’s like a picnic and she doesn’t know they’re animaguses yet SO SHE’S LIKE “HEY, LOOK, A DOG!” and he totally takes over the date and she’s like rubbing his belly and patting his head and scratching his ears and just giving him all this attention like “who’s a good boy? who’s such a cute doggy? who’s a handsome boy?” and sirius is just giving james these smug looks who obviously KNOWS and he has to try to be interested in the dog too. but every time lily turns her back he’s giving sirius this look like “i’m GOING TO CLOBBER YOU?? WHAT IF I DID THIS TO YOU AND REMUS?” And sirius gives him a look like “try it, mate” and james i sjust fuming and lily haS no idea what is up with her weird boyfriend,,,,, kill me dead. kill me DEAD